1. Field of the Invention
The present invention relates to a printing medium suitable for use in ink-jet printing, and an ink-jet printing process and an image forming process using such a printing medium.
2. Related Background Art
An ink-jet printing method is a recording method in which recording is conducted by generating and ejecting droplets of an ink by one of various ink ejection systems, for example, an electrostatic attraction system, a system using a piezoelectric element to give an ink mechanical vibration or displacement, or a system in which an ink is heated to form bubbles in the ink, thereby using the pressure thus produced, and applying the droplets in whole or in part to a printing medium such as paper or a plastic film coated with an ink-receiving layer. The ink-jet printing method attracts attention as a printing method which produces minimal noise and can conduct high-speed printing and multi-color printing.
As inks for ink-jet printing, inks comprising water as a principal component are mainly used due to issues of safety, printability, etc. Water-soluble organic solvents such as polyhydric alcohols are often added to such inks with a view toward preventing clogging of orifices and improving ejection stability.
As printing media heretofore used in ink-jet printing, there are described, for example, a printing sheet comprising a water-insoluble polymer latex composed of a copolymer with a monomer having a tertiary amino group or quaternary ammonium group in Japanese Patent Application Laid-Open No. 57-36692, a printing sheet comprising an electric-conductive agent of the quaternary ammonium salt type in Japanese Patent Application Laid-Open No. 58-177390, a printing sheet comprising a diallyldialkylammonium halide in Japanese Patent Application Laid-Open No. 59-20696, and a printing sheet comprising a dicyandiamide-formalin condensate in Japanese Patent Application Laid-Open No. 59-146889.
Besides, there are described a printing sheet comprising a quaternary cationic or amine compound in Japanese Patent Application Laid-Open No. 61-277484, a printing sheet comprising polyallylamine hydrochloride in Japanese Patent Application Laid-Open No. 62-174184, a printing sheet comprising an organic acid salt of polyethyleneimine in Japanese Patent Application Laid-Open No. 59-198186, a printing sheet comprising a quaternized product of polyethyleneimine in Japanese Patent Application Laid-Open No. 59-198188, a printing sheet comprising a poly(dialkanolallylamine) derivative in Japanese Patent Application Laid-Open No. 63-280681, a printing sheet comprising a polymer based on a (meth)acrylic acid alkyl quaternary ammonium salt or a polymer based on a (meth)acrylamidoalkyl quaternary ammonium salt in Japanese Patent Application Laid-Open No. 63-115780, and a printing medium comprising a polyvinyl acetal resin and a cationic compound as essential components in Japanese Patent Application Laid-Open No. 7-61113.
Furthermore, there is also proposed an additive for ink-jet printing comprising, as an active ingredient, a polymer based on a (meth)acrylic acid alkyl quaternary ammonium salt having a benzyl group or a polymer based on a (meth)acrylamidoalkyl quaternary ammonium salt having a benzyl group in Japanese Patent Application Laid-Open No. 8-108618.
With the improvement in performance of ink-jet printing apparatus, such as speeding up of printing and multi-coloring of images, in recent years, ink-jet printing media have also been required to have improved properties. Particularly, the printing media are required to have the following five properties:
(1) being able to stably store an image formed thereon for a long period of time without deterioration in a high-temperature and high-humidity environment; PA1 (2) providing a printed image having excellent light fastness; PA1 (3) having high ink absorptivity (absorbing capacity being great, and absorbing time being short); PA1 (4) providing dots high in optical density and clear in periphery; and PA1 (5) having an ink-receiving layer excellent in water resistance and providing a printed image excellent in water fastness. PA1 (6) being excellent in adhesion between an ink-receiving layer and a base material; PA1 (7) providing dots having a substantially round shape and a smooth periphery; PA1 (8) undergoing little change in properties even at varied temperatures and humidities and undergoing no curling; PA1 (9) undergoing no blocking; and PA1 (10) being stable without undergoing deterioration even when stored for a long period of time (particularly, in a high-temperature and high-humidity environment). PA1 1) it has a functional group the quaternized nitrogen atom of which has an aromatic ring, and PA1 2) it has an alkyl group the main chain of which has 10 to 30 carbon atoms.
In addition to the above properties, such printing media are required to satisfy the following properties at the same time:
Besides, printing sheets for OHP, and the like, are further required to have excellent transparency in addition to the above requirements. More specifically, not only a film as a base material but also an ink-receiving layer provided thereon is required to have excellent transparency.
On the other hand, for white base materials such as white films and resin-coated paper, ink-receiving layers provided thereon are also required to have excellent transparency so as not to impair the whiteness and/or the glossy feeling of the base materials themselves. With respect to glossiness in particular, it is a matter of course that the glossiness of an unprinted portion of an opaque printing medium be high, and it is also necessary for a printed portion to have high glossiness.
These properties are often in a relation of trade-off. It has hence been impossible to satisfy them at the same time by the conventionally known techniques.
Especially, as ink-jet techniques become more widespread, opportunities for printing, storing and posting at various places are increasing. Therefore, discoloration or bleeding of printed images becomes a serious problem upon exposure to temperature, humidity or sunlight.
In order to obtain prints comparable to a silver salt photograph by ink-jet printing, there is a demand for development of a printing medium capable of providing an image with brighter colors.
As the exemplified printing media of the prior art, for example, the printing sheets comprising a cationic compound proposed in Japanese Patent Application Laid-Open Nos. 57-36692, 58-177390, 59-20696, 59-146889, 61-277484, 62-174184, 59-198186, 59-198188, 63-280681, 63-115780 and 7-61113 are recognized to provide an image greatly improved in water fastness compared with any printing sheet without an addition of a cationic compound when conducting ink-jet printing. However, the resultant image on each printing sheet undergoes a change in the hue of an ink-jet printed portion though it somewhat varies according to the kinds of dyes used, so that the color of the image becomes greatly different from the hue inherent in the dye, or the image becomes a dull image having poor brightness. The reason for it is considered to be attributable to the fact that the cationic compound is ionically bonded to the dye to form a great polymeric complex, so that the aggregating state of the dye undergoes a change, and the light absorption spectrum inherent in the dye is hence varied. Further, in printing sheets obtained by providing an ink-receiving layer on a support, such as sheets for OHP, images printed thereon bleed when left to stand in a high-temperature and high-humidity environment (for example, 30.degree. C./80% RH) even if such a cationic compound as proposed in the prior art is used.
With the increase of printing density in ink-jet printing methods in recent years, i.e., increase in shot-in ink quantity, the resistance to bleeding of an image when the image is left to stand in a high-temperature and high-humidity environment (for example, 30.degree. C./80% RH) comes to be unsatisfactory under the circumstances though it has heretofore been at an entirely satisfactory level.
Further, the printing sheets proposed in the prior art provide an image markedly poor in light fastness compared with printing sheets containing no cationic compound, and so the necessity of improving the light fastness of the resulting image is pointed out.
When a printing medium for ink-jet is prepared by mixing the additive for ink-jet printing proposed in Japanese Patent Application Laid-Open No. 8-108618 with a hydrophilic resin and applying the mixture to a transparent PET film to form an ink-receiving layer, and ink-jet printing is conducted thereon, an image, which is entirely satisfactory in coloring and light fastness, is provided. However, the ink-jet printed image is not wholly satisfactory as to resistance to bleeding when the image is left to stand in a high-temperature and high-humidity environment (for example, 30.degree. C./80% RH).